So far, our examples have been simple standalone subprograms. Ada is helpful in
that regard, since it allows arbitrary declarations in a declarative part. We
were thus able to declare our types and variables in the bodies of main
procedures.

However, it is easy to see that this is not going to scale up for real-world
applications. We need a better way to structure our programs into modular and
distinct units.

Ada encourages the separation of programs into multiple packages and
sub-packages, providing many tools to a programmer on a quest for a perfectly
organized code-base.

with Ada.Text_IO; use Ada.Text_IO;
with Week;
-- References the Week package, and adds a dependency from Main
-- to Week
procedure Main is
begin
Put_Line ("First day of the week is " & Week.Mon);
end Main;

Packages let you make your code modular, separating your programs into
semantically significant units. Additionally the separation of a package's
specification from its body (which we will see below) can reduce compilation
time.

While the with clause indicates a dependency, you can see in the example
above that you still need to prefix the referencing of entities from the Week
package by the name of the package. (If we had included a "use Week" clause,
then such a prefix would not have been necessary.)

Accessing entities from a package uses the dot notation, A.B, which is
the same notation as the one used to access record fields.

A with clause can only appear in the prelude of a compilation unit
(i.e., before the reserved word, such as procedure, that marks the
beginning of the unit). It is not allowed anywhere else. This rule is only
needed for methodological reasons: the person reading your code should be able
to see immediately which units the code depends on.

In other languages

Packages look similar to, but are semantically very different from, header
files in C/C++.

The first and most important distinction is that packages are a language-level
mechanism. This is in contrast to a #include'd header file, which is a
functionality of the C preprocessor.

An immediate consequence is that the "with" construct is a semantic
inclusion mechanism, not a text inclusion mechanism. Hence, when you
"with" a package, you are saying to the compiler "I'm depending on this
semantic unit", and not "include this bunch of text in place here".

The effect of a package thus does not vary depending on where it has been
"with"ed from. Contrast this with C/C++, where the meaning of the
included text depends on the context in which the #include appears.

This allows compilation/recompilation to be more efficient. It also
allows tools like IDEs to have correct information about the semantics
of a program. In turn, this allows better tooling in general, and code
that is more analyzable, even by humans.

An important benefit of Ada "with" clauses when compared to #include is
that it is stateless. The order of "with" and "use" clauses does not matter,
and can be changed without side effects.

In the GNAT toolchain

The Ada language standard does not mandate any particular relationship
between source files and packages; for example, in theory you can put all
your code in one file, or use your own file naming conventions. In
practice, however, an implementation will have specific rules. With GNAT,
each top-level compilation unit needs to go into a separate file. In the
example above, the Week package will be in an .ads file (for Ada
specification), and the Main procedure will be in an .adb file
(for Ada body).

As we have seen above, the with clause indicates a dependency on another
package. However, every reference to an entity coming from the Week package
had to be prefixed by the full name of the package. It is possible to make
every entity of a package visible directly in the current scope, using the
use clause.

In fact, we have been using the use clause since almost the beginning of
this tutorial.

with Ada.Text_IO; use Ada.Text_IO;
-- ^ Make every entity of the Ada.Text_IO package
-- directly visible.
with Week;
procedure Main is
use Week;
-- Make every entity of the Week package directly visible.
begin
Put_Line ("First day of the week is " & Mon);
end Main;

In the simple example above, the Week package only has
declarations and no body. That's not a mistake: in a package specification,
which is what is illustrated above, you cannot declare bodies. Those have to be
in the package body.

with Ada.Text_IO; use Ada.Text_IO;
-- ^ Make every entity of the Ada.Text_IO package
-- directly visible.
with Week;
procedure Main is
use Week;
-- Make every entity of the Week package directly visible.
begin
Put_Line ("First day of the week is " & Mon);
end Main;

with Ada.Text_IO; use Ada.Text_IO;
-- ^ Make every entity of the Ada.Text_IO package
-- directly visible.
with Week;
procedure Main is
use Week;
-- Make every entity of the Week package directly visible.
begin
Put_Line ("First day of the week is " & Mon);
end Main;

Here we can see that the body of the Increment_By function has to be
declared in the body. Coincidentally, introducing a body allows us to put the
Last_Increment variable in the body, and make them inaccessible to the
user of the Operations package, providing a first form of encapsulation.

This works because entities declared in the body are only visible in the
body.